Conventional schemes for conducting video communications by way of a satellite link use analog modulation formats, which require a very large information bandwidth in order to achieve the full motion and resolution that is characteristic of `broadcast quality` video. Due to international restrictions and FCC regulations placed upon satellite transmission power spectral density, it is necessary to use a physically large receive antenna with these traditional wideband analog modulation formats, in order to achieve the high signal-to-noise ratio associated with broadcast quality video.
Another factor that mandates the use of a physically large video receive antenna is the need to reject (interfering) transmissions from other satellites which are near the satellite sourcing the video. Because commercial satellites can be spaced as closely as 2.degree. (in longitude) from one another, the antennas utilized in the receive link from these satellites are typically designed to have a null-to-null beamwidth of less than 4.degree. (+/-2.degree.). Such a narrow beamwidth requires a considerably large antenna aperture at the allocated commercial satellite operating frequencies.
Unfortunately, the need to install a large geometry antenna on the aircraft is one of the greatest obstacles incurred to date in attempting to receive broadcast quality video from satellites. This has generally rendered the antenna, and therefore the communication system, to be impractical, because of size, cost, power and/or weight constraints associated with the aircraft. Indeed, the use of a purely mechanically steered aircraft antenna for this application is generally precluded, since most aircraft have limited space on board and the fact that a mechanically steered antenna requires a volume larger than that of the antenna itself, in order to accommodate steering over the range of pointing angles required to maintain communications during normal aircraft flight maneuvers.
To significantly reduce the volume required and to allow placement of the antenna on or near the aircraft's skin, an electronically steered (phased array) antenna (or one that is at least partially electronically steered) is preferred. Electronic scanning, however, affects the antenna aperture area required, since the gain of a phased array antenna configuration decreases (the beamwidth widens) as the antenna is electronically scanned off-boresight. For example, at a scan angle of 60.degree., the gain may drop by approximately 5 dB from what is achievable at boresight. This reduction in gain must generally be compensated by an increase in antenna aperture area (e.g. by a factor of more than three to recoup the five dB loss). Hence, although the phased array antenna occupies a much smaller volume than a mechanically steered antenna, there is still a strong incentive to reduce the required antenna aperture.